The invention relates to a collar mounted downhole tool.
One technique used to drill a wellbore involves rotational drilling in which a drill string is rotated to actuate a drill bit at the remote end of the drill string. The rotating bit cuts through subterranean formations opening a path for the drill pipe that follows. Another technique involves using a motor, as opposed to rotating the drill string, to actuate the drill bit. The motor responds to drilling fluid that is forced through a central passageway of the drill string to the motor. The drilling fluid exits the motor and returns to the surface via an annular space, or annulus, that is located between the drill string and the wellbore.
It is usually desirable to obtain information about one or more downhole conditions as drilling progresses. For example, it may be desirable to know the wellbore inclination angle, wellbore magnetic heading and/or the tool-face orientation of the bottom-hole assembly to ensure that drilling is progressing in the right direction. Other useful information includes radioactivity of the formation to discriminate between sands and shale, resistivity and porosity of the formation to determine if oil is present.
These downhole conditions are typically measured by sensors located as near as possible to the bit. A downhole measurement while drilling (MWD) mud pulser transmits these measurements to the surface of the well by modulating the already present stream of drilling fluid that circulates down the central passageway of the drill string and up through the annulus. Sensor measurements are typically encoded in the stream by selectively restricting the flow of drilling fluid. As a result of these restrictions, the encoded data takes on the form of pressure pulses. Sensors at the surface of the well decode these pressure pulses to recover the downhole information from the mud stream.